Cobalt base alloys



United States Patent 3,346,378 CQBALT BASE ALLOYS Allan D. Foster, Schenectady, and Chester T. Sims, Ballston Lake, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed Mar. 22, 1965, Ser. No. 441,331 11 Claims. (Cl. 75171) ABSTRACT OF THE DISfiLOSURE High temperature oxidation and corrosion resistant cobalt base alloys useful in high temperature operating equipment have a percent by weight content consisting essentially of carbon 0.10 to 0.60, chromium 24.0 to 35.0, tungsten 6.0 to 9.0, nickel 8.5 to 11.5, iron 6.0 maximum, boron 0.50 maximum, yttrium 0.01 to 1.0, manganese 1.0 maximum as impurity, with the remainder essentially cobalt.

This is a continuation-in-part of application Ser. No. 415,507 filed Dec. 2, 1964. The invention relates to new and useful alloys. More particularly, it relates to alloys which possess suitable high temperature strength and are resistant to oxidizing, sulfidizing and other corrosive combustion gases at elevated temperatures.

It is well known that machinery depending upon the driving force of combustion gases, such as gas turbines, operate more eti'iciently and with greater power output at elevated temperatures. It is also well known that at such elevated temperatures the strength of many materials often decreases drastically and that they tend to become subject to excessive corrosion caused by oxidizing, or other corroding constituents in the atmospheres or fuels burned. Such constituents could be sulfur, sodium vanadium, and others. As the operating temperature of such equipment rises, relatively small improvements in the corrosion resistance and strength of such materials become important. In gas turbines operating at average temperatures of the order of about 1600 F. with peak temperatures up to about 2000 F., an improvement of only one hundred degrees Fahrenheit in the corrosion resistance of the materials of construction represents a notable advance. For example, in a typical gas turbine an increase in operating temperature from about 1500 F. to about 1600 F. represents an increase in output power of about 14% and an increased efiiciency of one to five percent. It is, therefore, a principal object of this invention to provide new and useful alloys which will permit substantial increases in the operating temperatures of such equipment as gas turbines. Another object of the invention is to provide improved materials of construction for other high temperature equipment subjected to oxidizing and/orcorrosive atmospheres such as furnaces and the like.

Briefly, there are provided by the present invention high-temperature corrosion-resistant cobalt-base alloys having percent by weight content of carbon 0.10 to 0.60, chromium 24.0 to 35.0, tungsten 6.0 to 9.0, nickel 8.5 to 11.5, iron 6.0 maximum, boron 0.050 maximum, yttrium 0.01 to 1.0, manganese 1% maximum as impurity not to be added, with the remainder cobalt except for incidental impurities such as phosphorus, sulfur, silicon and the like. It has been found that alloys having the above carefully controlled metallurgical composition are characterized by substantial increases in corrosion resistance, while at the same time retaining suitable rupture strength and ductility for high temperature operation. The materials are also particularly useful in that they are readily weldable, permitting the fabrication of various shaped structures.

Not only has it been found that compositions as above containing from about 24 to 26.5% chromium, along with the other stated materials particularly including yttrium, are salutary in their character, but it has also been unexpectedly found that such alloys containing from about 26.5 to 35% chromium are most useful in the present respect. This is directly contrary to the teaching of the prior art which teaches that alloys having a chromium content of over about 25% by weight show an increase in scaling or deterioration at elevated tempera tures. This prior art teaching is set forth, for example, in Journal of the Electrochemical Society, volume 103, No. 8, by Pfalnikar et' a1., entitled, High Temperature Scaling of Cobalt-Chromium Alloys. It will thus be seen that the present invention produces results which are unexpected not only with respect to the lower ranges of chromium content, but as well for higher ranges of chromium content which proceeds directly in the face of the prior art teaching. The present invention also provides additional improvements over those conferred by yttrium additions alone.

As pointed out above, the present compositions comprise a carefully formulated combination of constituents, each of which contributes to the desirable characteristics obtained. Deviations in the proportions of the materials destroy this critical balance, and result in materials which have been found to be wanting in essential characteristics. For example, when the carbon content is lowered beyond that prescribed, an undesirable loss of strength results. On the other hand, increasing the carbon content above that stated detracts from the weldability of the material, as Well as rupture ductility, and lowers the oxidation resistance. Reduction of the chromium content below that set forth results in a detrimental loss of oxidation resistance. The nickel, iron and tungsten additives do not appear to be particularly critical for oxidation and corrosion resistance but it has been found desirable to have them present in the stated ranges for suitable mechanical properties. Boron imparts ductility to the alloy where required. Increasing the boron content beyond that set forth causes detrimental low-melting phases to form in the alloy. Yttrium, when used in lower amounts than that set forth above, results in a decreased oxidation resistance. As a practical matter, amounts of yttrium greater than the upper prescribed limit are difficult to retain effectively through melting into the product. The cobalt base, of course, is well known for its contribution to sulfidation and oxidation resistance. Manganese can be tolerated in amounts up to about 1.00%, but it is not intentionally added in any case. Other impurities, such as phosphor and sulfur, are held to a maximum of about Example 1 There was prepared an alloy consisting of, by weight percent from analysis of the cast sample, carbon 0.22, chromium 24.4, nickel 10.8, tungsten 7.7, iron 1.2, boron 0.0066, yttrium 0.15, manganese 1.02, with about 0.007 of phosphorus and 0.009 sulfur as impurities with the remainder essentially cobalt. This alloy was cast into pieces having dimensions of 1% inches diameter by 5 inches long from which one inch diameter discs 0.060 inch thick were cut for testing, and compared with a typical prior art alloy having a percent by Weight content of carbon 0.23, chromium 25.5, nickel 10.5, tungsten 6.9, iron 0.82, boron 0.007, with 0.54 manganese, 0.02 each of phosphor and sulfur as impurities and with the remainder essentially cobalt. It will be noted that this prior Q .3 art alloy is lacking in yttrium, which it has been found imparts unexpected and advantageous qualities to the present alloy.

When test pieces of the material of the above example the notable advance in corrosion resistance. The rupture ductility of the materials represented in the table also compares favorably with prior art material.

There are provided, then, by the present invention, new

and the above prior art material were tested edgewise to and useful alloys Which are particularly characterized by the combustion gas stream flow in a small burner appatheir high strength at elevated temperatures and their ratus at a temperature of 2000 F. burning natural gas ability to withstand at such elevated temperatures corwith an air-to-fuel Weight ratio of 50 to 1, the corrosion rosive attack as from oxidizing and sulfidizing combusin mils per side of the prior art material after 594 hours tion gas constituents. They are particularly useful for was 14.6. On the other hand, the material of the above 10 fabricating gas turbine parts such as nozzle partitions, as example had corresponding corrosion of only 5.7 mils per well as parts of other equipment which are exposed to side. After 984 hours the prior art material exhibited high temperature corrosive and/or oxidizing conditions. corrosion of 20.4 mils per side and after even 2431 hours They are also useful in general in furnaces for fans, the material of Example 1 exhibited corrosion of only 15.9 runners, and in other equipment which operate under simmils per side. It will thus be seen that the present above ilar rigorous conditions.

alloy exhibits about a three-fold increase in oxidation What we claim as new and desire to secure by Letters resistance at 2000 F. The material of the above example Patent of the United States is:

was tested under sulfidizing conditions, as was the above 1. A high temperature resistant alloy consisting essenprior art material, in a test burner operating at a temtially of by weight carbon 0.10 to 0.60 percent, chromium perature of 1600 F. with an air-to-fuel Weight ratio of 24.0 to 26.5 percent, tungsten 6.0 to 9.0 percent, nickel 68 to 1, burning a distillate oil containing 3.8% by Weight 8.5 to 11.5 percent, boron 0.050 percent maximum, yttrium of sulfur to which 325 arts per million of sodium chloride 0.01 to 1.0 percent, iron 6.0 percent maximum with the had been added. In this type of test, deposition of molten remainder essentially cobalt.

material can occur on the leading edge of the test speci- 2. A high temperature resistant alloy consisting essenmen, causing a higher penetration rate than where the tially of by weight carbon 0.22 percent, chromium 24.4 deposition does not occur. The data presented here are percent, tungsten 7.7 percent, nickel 10.8 percent, boron both the minimum attack (no molten deposition) and the 0.0066 percent, yttrium 0.15 percent, iron 1.2 percent, maximum attack where molten material resided. After with the remainder essentially cobalt.

465 hours of such testing, the typical prior art material 3. A high temperature resistant alloy consisting essenhad a corrosion in mils per side of the test specimen rangtially of by weight carbon 0.10 to 0.60 percent, chromium ing from about 3.8 (minimum) to 18.1 mils (maximum). 24.0 to 35.0 percent, tungsten 6.0 to 9.0 percent, nickel On the other hand, the material of Example 1, even after 8.5 to 11.5 percent, boron 0.050 percent maximum, yttrium 584 hours of operation under such rigorous conditions, 0.01 to 1.0 percent, iron 6.0 percent maximum, with the had only about 1.8 (minimum) to 6.9 mils (maximum) remainder essentially cobalt.

per side of corrosion. 3 4. A high temperature resistant alloy consisting essen- As pointed out above, not only are the alloys of the tially of by weight carbon 0.10 to 0.60 percent, chromium present invention characterized by good oxidation and 26.5 to 35.00 percent, tungsten 6.0 to 9.0 percent, nickel sulfidizing resistance, but their fabrication into various 8.5 to 11.5 percent, boron 0.050 percent maximum, yttrishaped structures is facilitated by the fact that they are um 0.01 to 1.0 percent, iron 6.0 percent maximum with readily Weldable. 40 the remainder essentially cobalt.

The rupture strength of the material of Example 1 at 5. A high temperature resistant alloy consisting essen- 10 hours at a temperature of 1600 F. is about 10,500 tially of by weight carbon 0.23 percent, chromium 28.9 p.s.i., which is the same value obtained from prior art percent, tungsten 8.4 percent, nickel 10.4 percent, boron material. The rupture ductility of the above exemplary 0.001 percent, yttrium 0.05 percent, iron 0.65 percent, with material also compares favorably with prior art material. the remainder essentially cobalt.

Example 1 was repeated in all respects except for the 6. A high temperature resistant alloy consisting essenalloy constituent content and the alloys tested as shown tially of by weight carbon 0.24 percent, chromium 29.6 inthe table below: percent, tungsten 8.4 percent, nickel 10.4 percent, boron TABLE Oxidtatggs OForrosion, Stress-Rupture Behavior Ex. Composition (Wt. percent) Chemical Analysis I Rupture Time to Time, Corrosion, Strength, Temp, Failure, C Cr Y Ni W Fe Mn B 00 Hrs. mils/side p.s.i. F. Hrs. 2 0. 23 28.9 0. 05 10.4 8.4 0. 0.40 0.0010 Balk--- 635 20,000 1,450 331.4 5. 9 10, 000 1, 500 1, 726. 0 5,000 1, 800 1, 490. s 3 0. 24 29.6 0.38 10.4 8.4 0. 70 0.41 0. 002 Ball." 635 4' 5 20,000 1,450 505.3 10. 000 1,600 1, 075. 6 4 0. 24 28.6 0. 39 10.5 8.3 0. 0.41 0.0011 Bal. 609 20,000 1,450 400.1 10,000 1, 600 970. 4 5 0.26 31.0 0.17 8.5 7.9 0.72 0. 29 0. 0025 Ball..- 15, 000 1,600 35.2

1 Lesser constituents and impurities from common starting materials About. about as follows: Phosphorous 0.02 wt. percent. 4 Not tested.

2 Balance essentially cobalt.

From the above table it Will be noted that compositions about 0.002 percent, yttrium 0.38 percent, iron 0.70 perof the present invention containing relatively higher Cent, will} the remainder essfimtially whaliamounts of chromium produce alloys which exhibit excep- A f l i i ig z z alloyt cogslstiflg 5g; 1a y o we1g car on percen c romium 'nonauy f corroswn'reslbtance elevated f percent, tungsten 8.3 percent, nickel 10.5 percent, boron tures. It will also be noted that the stress-rupture behavior @0011 percent, yttrium 039 percgnt, iron 066 Percent as set forth is equivalent to prior art material, even with with the remainder essentially cobalt.

8. A high temperature resistant alloy consisting essentially of by Weight carbon 0.26 percent, chromium 31.0 percent, tungsten 7.9 percent, nickel 8.5 percent, boron 0.0025 percent, yttrium 0.17 percent, iron 0.72 percent, with the remainder essentially cobalt.

9. A high temperature resistant structure comprising the alloy of claim 1.

10. A high temperature resistant structure comprising the alloy of claim 3.

11. A high temperature resistant structure comprising 10 the alloy of claim 4.

6 References Cited UNITED STATES PATENTS 2,744,010 5/1956 Callaway 75171 2,746,860 5/1956 Binder et a1. 75--171 2,855,295 10/1958 Hansel 7517l 2,996,379 8/1961 Faulkner 75-171 3,202,506 8/1965 Deutsch 75-171 HYLAND BIZOT, Primary Examiner. RICHARD O. DEAN, Examiner. 

3. A HIGH TEMPERATURE RESISTANT ALLOY CONSISTING ESSENTIALLY OF BY WEIGHT CARBON 0.10 TO 0.60 PERCENT, CHROMIUM 24.0 TO 35.0 PERCENT, TUNGSTEN 6.0 TO 9.0 PERCENT, NICKEL 8.5 TO 11.5 PERCENT, BORON 0.050 PERCENT MAXIMUM, YTTRIUM 0.01 TO 1.0 PERCENT, IRON 6.0 PERCENT MAXIMUM, WITH THE REMAINDER ESSENTIALLY COBALT. 